1. Field of the Invention
The present invention generally relates to an apparatus and method for processing material using lasers and more particularly to removal of material using lasers and more particularly to removal of material using a laser with optimally configured pulses.
2. Description of the Related Art
Lasers and laser systems for drilling materials are commercially available. Many of these lasers typically cause excessive heating of the material on which they are incident and this may not be desirable for certain types of material processing. Other commercially available lasers typically produce Q-switched or mode-locked pulses and it is difficult to control the duration, shape and spacing of the pulses generated by such lasers.
In principle, it is possible to control the pulse shape by using an active modulator in the laser resonator, an approach used, for example in the DP-11 laser available from TRW Inc. The DP-11 laser can produce a series of short (xcx9c100 ns), spaced ( greater than 20 xcexcsec) pulses under the xcx9c400 xcexcs long diode pump pulse envelope.
However, the use of a modulator in such an actively controlled system results in certain drawbacks such as increase in the cost and complexity of the system and power and efficiency limitations. Specifically, the DP-11 laser generates 350-500 W average power when unmodulated (400 Hz repetition rate quasi-CW operation with 400-500 xcexcs long pulses), whereas the output power is limited to xcx9c80 W with the modulated pulse format. Further, such a modulator is not usable at high power since the active modulator device probably cannot be scaled. The DP-23 laser, also available from TRW Inc., is more powerful (nominally 2-3 kW average with 400 xcexcs long pulses), but does not use such modulation.
Electro-Discharge Machining (EDM) is also presently used for drilling small holes. However, duration times on the order of tens of seconds or longer are required for each hole and diameters smaller than xcx9c0.006xe2x80x3 and depth to diameter ratios greater than xcx9c50 are very difficult to achieve.
Phase conjugate master oscillator power amplifier laser architectures (PC-MOPA) capable of achieving high average output power with near diffraction-limited beam quality are known. See, for example, the article xe2x80x9cA Review of Phase-Conjugate Solid-State Lasersxe2x80x9d by David A. Rockwell, IEEE Journal of Quantum Electronics, vol. 24, no. 6, June 1988, pp. 1124-1140, the content of which is hereby incorporated herein by reference.
Loop phase conjugate mirrors (Loop PCMs) are also known. See, for example U.S. Pat. No. 5,729,380 entitled xe2x80x9cLoop Phase-Conjugate Mirror For Depolarized Beamsxe2x80x9d, inventors: Alexander A. Betin and Metin S. Mangir, issued Mar. 17, 1998 to Hughes Electronics Corporation, the assignee of the present invention. Also see U.S. Pat. No. 5,726,795, entitled xe2x80x9cCompact Phase-Conjugate Mirror Utilizing Four-Wave Mixing In a Loop Configurationxe2x80x9d, inventors: Alexander A. Betin, Metin S. Mangir and David A. Rockwell, issued Mar. 10, 1998, and assigned to Hughes Electronics Corporation, the assignee of the present invention. The subject matter of U.S. Pat. Nos. 5,726,795 and 5,729,380 are incorporated herein by this reference.
Additional information regarding loop PCM can be found in A. A. Betin and O. V. Mitropol""sky, xe2x80x9cGeneration of radiation by four-wave interaction in a feedback system in the xcex=10 xcexcm range,xe2x80x9d Sov. J. Quant.Electron. 17, 636 (1987) and the article by A. S. Dement""ev and E. Ya. Murauskas, xe2x80x9cEmission from a YAG:Nd laser with a four-wave thermal mirror in a ring resonator,xe2x80x9d Sov. J. Quant. Electron. 18, 631 (1988), each of which is hereby incorporated by reference herein.)
FIG. 1 is a diagram of a system employing Loop PCM. The input beam E1100 first passes through a nonlinear medium 102, which can be a simple absorption cell 344. The input beam 100 is then directed through an amplifier 104 having a gain G by two or more mirrors 106, 108 to form a loop or ring. The amplified wave 110, labeled E3, is directed to intersect E1 100 at a small angle in the cell 102. These propagating waves, having sufficient coherence length, form an interference pattern in the nonlinear medium 102 that produces an associated index grating of modulation dn xcx9cE1E3*. The grating is characterized by a reflectivity R which closes the loop and allows ring laser oscillation under the condition RG greater than 1. Not shown, but used in many cases, is a non-reciprocal optical diode that prevents saturation of the loop amplifier 104 by the incoming input beam 100 and preferentially selects the ring oscillation to be in the opposite direction from the input beam 100. Being the laser oscillation mode, beam E2 112 starts from spontaneous noise, diffracts from the grating to become beam E4 114 and is amplified as it passes around the loop and becomes E2 112 again. The grating and loop resonator select wave E2 112 to be phase conjugated to the input beam 100. The portion of E2 112 that is transmitted by the grating is, finally, the output wave Eout 116, which is phase conjugate to E1 100 and may be larger in amplitude.
Any kind of nonlinear mechanism for recording a grating hologram can be used, but most of the work reported in the literature has been done using the thermal nonlinearity in liquids and the gain saturation effect in the active medium of the amplifier itself. References which discuss the use of thermal nonlinearity in liquids include: A. A. Betin and O. V. Mitropol""sky, xe2x80x9cGeneration of radiation by four-wave interaction in a feedback system in the xcex=10 xcexcm range,xe2x80x9d Sov. J. Quant.Electron. 17, 636 (1987); A. S. Dement""ev and E. Ya. Murauskas, xe2x80x9cEmission from a YAG:Nd laser with a four-wave thermal mirror in a ring resonator,xe2x80x9d Sov. J. Quant. Electron. 18, 631 (1988); A. A. Betin and A. V. Kirsanov, xe2x80x9cSpatial structure of radiation from a neodymium glass four-wave feedback oscillator,xe2x80x9d Sov.J.Quant.Electron. 22, 715 (1992); A. A. Betin and A. V. Kirsanov, xe2x80x9cSelection of a phase-conjugate wave in an oscillator based on a four-wave interaction with feedback in an extended nonlinear medium,xe2x80x9d Quantum Electronics 24, 219 (1994); K. V. Ergakov and V. V. Yarovoy, xe2x80x9cEnergy optimization of an Nd:YAG-based four-wave-mixing oscillator with feedback and investigation of its adaptive properties in the pulse-periodic regime,xe2x80x9d Quantum Electronics 26, 389 (1996), each of which is hereby incorporated by reference herein. References which use the gain saturation effect in the active medium of the amplifier itself include: A. A. Betin, xe2x80x9cPhase conjugation based on thermal nonlinearity,xe2x80x9d Nonlinear Optics, Maui, Hawaii, July 1996, Techn. Digest v. 11, p.336-339; R.P.M. Green, G. J. Crofts and M. J. Damzen, xe2x80x9cHolographic Laser resonators in Nd:YAG, xe2x80x9cOptics Letters 19, 393 (1994); A. V. Berdyshev, A. K. Kurnosov, and A. P. Napartovich xe2x80x9cFormation of amplitude grating in the medium of a CO laser subject to the field of its own multifrequency radiation,xe2x80x9d Quantum Electronics 24 87 (1994); A. A. Ageichik, et al. xe2x80x9cSelf-phase conjugation of middle-infrared radiation by four-wave mixing in active medium of CO2 laser with feedback loop,xe2x80x9d in Laser Optics ""95: Phase Conjugation and Adaptive Optics, Vladimir E. Sherstobitov, Editor, Proc. SPIE 2771, 119-125 (1996), each of which is hereby incorporated by reference herein.
There is a continuing need for laser systems and methods that improve the efficiency of laser materials processing, particularly with respect to material removal such as laser drilling.
To address the requirements described above, the present invention provides an apparatus for generating radiation comprising a master oscillator and a phase conjugator for controlling transient relaxation oscillations to form sustained pump pulses. In a preferred embodiment, the phase conjugator is a loop phase conjugate mirror (Loop PCM). The present invention uses the oscillatory output of the Loop-PCM for material processing applications by controlling the pulsations and sustaining them, even for pump pulses exceeding 1 ms. Through selection of the pulse duration and spacing in accordance with the teachings herein, the pulses are made suitable for various materials processing applications.
In a preferred embodiment suitable for applications requiring greater amplification, the master oscillator is part of a phase conjugate master oscillator power amplifier (PC-MOPA).
The laser pulses generated by the present laser very effectively and efficiently remove material, providing nearly thousand-fold enhancements in drilling efficiency in some cases while using microsecond time scale laser pulses. The apparatus of the invention is capable of drilling holes using much less laser energy per hole than the related art, and, consequently, much higher hole production rates are possible with a given laser power.
The present invention can substantially reduce the laser power requirements to achieve specific desired material processing results in various laser applications, including medical surgery and military applications. Although the drilling of holes is discussed in some detail herein as an exemplary case, the scope of the present invention is not limited to drilling but is applicable to all types of material processing applications. A method for determining an optimal pulse energy, duration and spacing for any given material is described. Using pulses of the kind that are produced by the transient, and normally not desirable, relaxation oscillations common to neodymium yttrium aluminum garnet (Nd:YAG) and other lasers, the present PC-MOPA-Loop PCM can sustain relaxation oscillations with controllable pulse duration, repetition rate and duty cycle, making it ideal for materials processing.
Key aspects of and advantages achieved by the present invention include laser drilling using microsecond pulse formats at particular repetition rates; a method for selecting an optimal pulse energy, duration, and spacing for any given material; a unique laser architecture that makes it possible to achieve sustained relaxation oscillations, producing suitable pulses; and techniques to control the repetition rate, pulse duration and duty cycle (the repetition rate and pulse duration being interrelated but not independently controllable).
These and other features and advantages of the present invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings.